Hawk of Hawk & Dove became Monarch.
So did Captain Atom.
Adam Monroe of Heroes starts out as a (rather ineffectual) hero, then turns supervillain after a few centuries of boredom and frustration with humanity.
I’m not sure that Daimon Hellstrom counts, but similarly-backstoried Illyana Rasputin may.
Black Adam, somewhat equivocally.
The original Marvel Boy became the villainous Crusader.
Two heroes called Dynamic Man have been recast as villains (or at least extreme dicks).
The third Foolkiller–Kurt Gerhardt–was sort of heroic, but was one of the criminals who broke out of the Vault, and seemed pretty vicious.
Glamour and Illusion, a “Nick and Nora Charles”-type couple of stage magicians, were introduced as pals of Vision and Scarlet Witch, but later revealed to be jewel thieves.
Triumph, introduced as the forgotten founder of the Justice League, was last seen betraying them. He is currently petrified in their trophy room.
Congorilla!
If memory serves: Yellowjacket, in despair after his wife left, got tricked into helping Egghead and then kind of went on a villainous rampage of his own for a while. But I think he got better.
Baldwin - I don’t much care for Shelley as a general rule, and thanks for the spelling tip. Some days, you know?
Thanks, everybody who’s posted so far. Skald, I’m keeping my eye on you and the horse you rode in on.
Trihs, I’m struggling with the idea of the Hulk being intelligent enough to be a villain. Banner, yes. That I could see.
The Hulk is frequently quite intelligent, occasionally every bit as intelligent as Bruce Banner.
Oh, I trust you both. I just never really read many Hulk comics and so my view is a little shaded towards the other mass-media versions.
I did not name the horse. I was informed of the horse’s preferred appellation when she became my steed. Admittedly I might have preferred a different one, but you do not argue with winged horses.
There have been several series–Brat Pack, Maximmotal, Marshal Law, The Boys, Top Ten– that make the convincing case that there is no real distinction between superheroes and super-villains.
Iron Man in the Civil War series.
Hank was mentally unstable for a long time…does his villainy count if he wasn’t really responsible for it?
Squadron Supreme.
Oh.
then you merely be allowed to wait tables, in the Officers’ Mess.